1. Field of the Invention
The present invention relates to a hologram recording medium such as an optical card with which information is optically recorded or reproduced, and more particularly, to a hologram recording medium which has a recording layer irradiated with an optical beam for recording information thereon or reproducing information therefrom, and a recording/reproducing method and a recording reproducing apparatus.
2. Description of the Related Art
A hologram has drawn attention because of its ability to record two-dimensional data at a high density, for use in high density information recording. The hologram is characterized by volumetrically recording a light wavefront, which carries recording information, on a hologram recording medium made of a photosensitive material such as a photo-refractive material as a change in refractive index. Multiplex recording on the hologram recording medium can dramatically increase the recording capacity. The multiplex recording is classified into angle multiplexing, phase coding multiplexing, and the like. Even in a multiplexed hologram region, information can be recorded multiple times by changing the incident angle or phase of interfering light waves.
A hologram recording method for increasing the recording density through a plurality of times of shift multiplex recording records holograms such that adjacent holograms are overlapped in sequence, as shown in FIG. 1 (see, Laid-open Japanese Patent Application No. 2002-40908). According to this method, the holograms are recorded in the form of sets of columns in which the centers of the holograms are spaced apart by 2 to 50% of their diameter from one another. In this recording method, a first set of holograms are recorded such that the respective holograms are tangential to each other (FIG. 1A). A second set of holograms are recorded on the first set of holograms such that the holograms in the second set are displaced from the holograms in the first set by 50% of their diameter, respectively (FIG. 1B). A third and a fourth set of holograms are recorded tangential to the first and second sets of hologram columns and are overlapped in a similar manner to the first and second sets of holograms (FIG. 1C). A fifth and a sixth set of holograms are recorded along a boundary between the first and second sets of hologram columns and the third and fourth sets of hologram columns, and are overlapped in a similar manner to the first and second sets of holograms (FIG. 1D).
In this conventional hologram recording method, when holograms are recorded such that they are overlapped at a narrow pitch equal to 50% of their diameter, the holograms are overlapped in excessively large portions. As the overlap writing is repeated a number of times, signals of adjacent holograms may be erased, or cross-talk may occur upon reading, though the method is intended to reduce a physical change problem. Data may not be successfully reproduced due to overwritten holograms. Further, since holograms cannot be precisely recorded in repetition, the holograms differ in pitch from one another, resulting in different diffraction efficiencies of the respective holograms.
On the other hand, an optical information recording apparatus has been developed for recording information at an ultra high density using the hologram recording medium as a disk (see, for example, Laid-open Japanese Patent Application No. 2003-85768). For recording an interference fringe pattern of a hologram, a proper exposure time and energy are required in a relative static state of the hologram recording medium and writing light, so that this prior art provides a method of continuing to precisely expose the moving hologram recording medium at a recording position thereof.
The conventional hologram recording medium includes servo areas 6 radially arrayed in a linear form at predetermined angular intervals, as shown in FIG. 2. Sector sections between adjacent servo areas 6 define data areas 7. For conducting a tracking servo control, the servo areas 6 and data areas 7 are formed with grooves on a track-by-track basis which function as guides. At least one lockup pit 8 has been previously recorded in the data area 7 in the form of an emboss pit or the like, so that even if an optical head is scanning the data area 7, a misalignment is detected between the lockup pit 8 and a position irradiated with a tracking servo light beam.
Likewise, with this optical information recording apparatus, when holograms are recorded, for example, such that they are overlapped at a narrow pitch equal to 50% of their diameter, signals of adjacent holograms may be erased, and cross-talk may occur upon reading, if the overlap writing is repeated a number of times.
Conventionally, hologram recording positions are sequentially displaced such that holograms overlap with one another to record the holograms in the form of a column, and the next hologram column is recorded to overlap the previous one, so that the previous overlapping holograms can cause a change in the intensity of next incident light, when it reaches the disk, thereby preventing optimal recording at constant recording power.
Particularly critical is a degraded quality of reproduced signals in adjacent hologram columns in a direction perpendicular to a direction in which the holograms extend. Also, the normal shift multiplexing hologram recording requires to precisely control a recording time for each hologram to be recorded. For this reason, a complicated control is required for maintaining a diffraction efficiency, including a recording scheduling arranged for an overall hologram recording medium.